The Link Between Wetland Ecology and Hydropedology

Posted on 10 Sep 2020

The landscape within South Africa and globally is becoming increasingly urbanised and developed into the infrastructure required for the various economic sectors. The development of previously open space, and thus relatively undisturbed land has an impact on the previously unquantified relationship between the soil-hydrology interface downslope and resultantly the wetland and/or river environments typically situated at the valley-bottom within the landscape. The wetlands and rivers that are at-risk of being impacted on by the ever-increasing developments are treated as isolated systems within the methodologies and techniques that are currently used to assess these ecosystems. The broader catchment areas from which the wetlands and rivers are fed have not yet been considered in the conservation and management of these valuable water resources. The field of hydropedology aims to define and utilise the soil-hydrology flow paths within the terrestrial component of the upslope wetland and river catchment areas to aid in the conservation and management of these previously isolated systems. 

 

Hydropedology is not a new science but has recently acquired national attention as its value is being realised at various spheres within government, specifically within the Department of Human Settlements, Water and Sanitation (DHSWS). The Water Research Commission (WRC) (2019) defines hydropedology as the study of the hydrological interaction of water with soil and the fractured rock zone. Its application essentially makes it possible to identify water resources and flow paths within the hillslope of a wetland from crest to the pinnacle, which is the wetland or stream itself. The flow diagram below by van Tol, le Roux and Lorents (2017) illustrates the processes and properties analysed when conducting a hydropedological study, the outputs and how they can be used in practice.

 


Figure 1: Flow diagram illustrating the inputs, outputs and uses of hydropedology (van Tol, le Roux and Lorents, 2017)

By studying the physical characteristics of the different soil forms, such as porosity and conductivity, as well as the morphological properties of the soil one can determine the dominant hydrological processes within the profile and in turn the hillslope. These hydrological processes, such as storage mechanisms, water flow paths and the connectivity between different flow paths, can then be characterised and conceptualised. The ability of the practitioner to spatially conceptualise these hydrological processes is a great benefit of the science of hydropedology, as this allows for more informed planning and management of water resources within the applicable catchment areas.  

 

The spatial conceptualization of the hillslope involves the classification of the various soil horizons and forms into five (5) hydrological soil types, namely: 1) Recharge, 2) Interflow between the A and B horizons, 3) Interflow between soil and bedrock, 4) Responsive, shallow soil and 5) Responsive, saturated soil (van Tol, le Roux and Lorents, 2013). Figure 2 below illustrates the typical hydrological flow paths (arrows) applicable to the different hydrological soil types. Recharge soils typically do not show any signs of wetness and they are well-drained soils in which the dominant flow direction is vertical through the soil and out of the profile into the underlying bedrock. Interflow (A/B) soils are those that have a less permeable B horizon, which facilitates the build-up of water in the topsoil and the lateral flow downslope at the interface between the horizons. Responsive soils typically do not encourage significant storage of water, and thus are characteristic by overland flow after rainfall events. The connectivity of the flow-paths in the example depicted in Figure 2 will result in the flow input into the downstream wetland/stream via the soil profile, or fractured bedrock.    

 


Figure 2: Typical hydrological flow paths (arrows) applicable to the several hydrological soil types (van Tol, 2019).

Each hydrological soil type has different hydrological responses depending on the soil properties and its position within the hillslope. Depending on what hydrological soil types are classified within a single hillslope, the  hillslope itself  can then be classed into one of six (6) different hillslope classes, namely: Class 1- Interflow (Soil/bedrock interface), Class 2- Shallow responsive, Class 3- Recharge to groundwater (not connected), Class 4- Recharge to wetland, Class 5- Recharge to mid-slope (typical of a hillslope seepage wetland) and Class 6- Quick interflow.

 

By classifying the hillslope, and thus the dominant hydrological processes within it, proposed development and/or conservation efforts on it or within the catchment area can be planned according to the potential impact/benefit it may have on the wetland/stream typically situated at the valley bottom. For instance, if a hillslope has been classified as a Class 1 (Interflow soil/bedrock) it can be assumed that a wetland situated at the valley bottom is primarily fed by lateral flow through the soil profile, atop the fractured bedrock. If a large opencast coal mine is planned within the section of the hillslope that has been classified as interflow hydrologic soils, the complete excavation of this section of the hillslope may result in the starvation of flow to, and destruction of, the wetland situated in the valley bottom. In addition to this, the fate of different pollutants can be estimated depending on what hydrological soil type it is spilled on. For example, if a spill were to occur on a recharge soil it is likely to flow vertically into the groundwater, or into the wetland at the valley bottom several months later via the fractured bedrock. However, if spilled on an interflow soil it will travel laterally in flow through the soil profile, which may result in the toxicant being filtered out by various chemical interactions with microorganisms in the soil.

 

Various other applications of hydropedology are evident in practice, which can drastically change the way hydrological processes within the soil profile are used to plan and coordinate numerous activities within the already stressed catchment areas of South Africa. The highly qualified specialist team at Environmental Assurance (ENVASS) recognize the importance and value of this exciting new field and its application. We have thus been involved in consultation with the DHSWS and various ‘founding fathers’ to refine our ability to undertake hydropedology studies for various sectors of the economy at a very high level. If ever the need arises, please do not hesitate to contact ENVASS for hydropedology studies, including: buffer zone determination, wetland/stream flow requirements, impact assessments, hillslope classification, land use planning and pollution migration assessments. You can contact us on 012 460 9768, or email us on info@envass.co.za. We look forward to hearing from you!